Land application of treated sewage sludge (often referred to as biosolids) for soil-amendment and land-reclamation purposes has increased over the past decade as a result of the ban on ocean dumping of wastewater residuals (Ocean Disposal Ban Act of 1988) and as an alternative to other disposal options, such as landfilling or incineration. Recycling sewage sludge has been practiced for many decades. In 1993, EPA promulgated Standards for the Use or Disposal of Sewage Sludge (Code of Federal Regulations Title 40, Part 503), which set pollutant limits, operational standards for pathogen and vector-attraction reduction, management practices, and other provisions intended to protect public health and the environment from any reasonably anticipated adverse effects from chemical pollutants and pathogenic organisms. Many of the regulations (commonly referred to as the Part 503 rule) were based on risk assessments conducted to identify and characterize risks associated with the use or disposal of sewage sludge. In this report, the National Research Council’s (NRC’s) Committee on Toxicants and Pathogens in Biosolids Applied to Land reviews the nature of the human health risks from chemicals and pathogens in biosolids; evaluates the scientific approaches that EPA used to establish its human-health-based land-application pollutant limits and pathogen reduction techniques; provides an overview of the advances in risk assessment since the establishment of those standards; and, in light of the advancements, recommends risk-based strategies for reevaluating the human-health-based land-application standards of the Part 503 rule.
This chapter briefly reviews why biosolids are a public-health concern, states the task addressed by the committee, sets forth the committee’s activities and deliberative process in developing the report, and describes the organization of the report.
Definitions and Use
Sewage sludge is defined in the Part 503 rule as the solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works. The term biosolids is not used in the Part 503 rule, but EPA (1995) defines biosolids as “the primarily organic solid product yielded by municipal wastewater treatment processes that can be beneficially recycled” as soil amendments. Use of the term biosolids has been controversial because of the perception that it was created to improve the image of sewage sludge in a public-relations campaign by the sewage industry (Rampton 1998). For the purposes of this report, the committee considers sewage sludge to be the solid, semi-solid, or liquid residue generated during treatment of domestic sewage, and biosolids to be sewage sludge that has been treated to meet the land-application standards in the Part 503 rule or any other equivalent land-application standards.
It is estimated that approximately 5.6 million dry tons of sewage sludge are used or disposed of annually in the United States, of which approximately 60% are used for land-application or public distribution (see Chapter 2). On the basis of data from EPA (1999a) and USDA (1997), EPA estimates that approximately 0.1% of available agricultural land in the United States is treated with biosolids. Biosolids are a complex mixture that may contain organic, inorganic, and biological pollutants from the wastewaters of households, commercial establishments, and industrial facilities and compounds added or formed during various wastewater treatment processes. Such pollutants include inorganic contaminants (e.g., metals and trace elements), organic contaminants (e.g., polychlorinated biphenyls [PCBs], dioxins, pharmaceuticals, and surfactants), and pathogens (e.g., bacteria, viruses, and parasites). Sewage-sludge treatment processes are intended to reduce the volume and organic content of biosolids and to reduce the presence of pathogens but retain beneficial properties for soil-amendment and land-reclamation purposes. Figure 1–1 provides a simplified schematic of how biosolids are produced and illustrates how the content of biosolids can vary depending on the wastewater streams and the variations in treatment processes. See Figures 2–1 and 2–2 in
BOX 1–1 Definitions
Sewage sludge: the solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works.
Chapter 2 for more detailed diagrams of wastewater and sewage sludge treatment.
Biosolids are applied to agricultural and nonagricultural lands as soil amendments, because they can improve the chemical and physical properties of soils and they contain nutrients and trace elements important for plant growth. Agricultural lands include sites where food crops (for human or animal consumption) and nonfood crops are grown. Nonagricultural lands include forests, rangelands, and public contact sites (e.g., public parks, golf courses, and cemeteries). Severely disturbed lands, such as strip mines and gravel pits, can be reclaimed with biosolids.
Biosolids are divided into two classes on the basis of pathogen content: Class A and Class B. Class A biosolids are treated to reduce the presence of pathogens to below detectable levels and can be used without any pathogen-related restrictions at the application site. Class A biosolids can also be bagged and sold to the public, if other requirements are met. Class B biosolids are treated to reduce pathogens but still contain detectable levels of them. Class B biosolids have site restrictions that seek to minimize the potential for human and animal exposure until environmental factors, such as heat, sunlight, and desiccation, have reduced pathogens further. Class B biosolids cannot be sold or given away in bags or other containers or used at sites with public use.
Sewage sludge that is not treated to meet land-application standards is usually disposed of at landfills or surface disposal sites that contain only sewage sludge or is incinerated. Regulations pertaining to these disposal practices are contained in the Part 503 rule. Review of disposal regulations is, however, outside the scope of the committee’s task.
Different methods were used to establish the chemical pollutant and pathogen standards in the Part 503 rule. For the chemical pollutant limits, sewage-sludge surveys (EPA 1982, 1990) and risk assessments (EPA 1992a,b) were used to identify and characterize risks from chemical pollutants in sewage sludge. The risk assessments considered a variety of pathways by which humans, animals, plants, and soil organisms could be exposed to biosolid pollutants. Chemical standards (i.e., ceiling concentrations (mg/kg), cumulative pollutant loading rates (kg/hectare), pollutant concentration limits (mg/kg), and annual pollutant loading rates (kg/hectare/365-day period) were originally established for 10 inorganic chemicals, using the most limiting exposure pathway. These chemicals are arsenic, cadmium, chromium,1 copper, lead, mercury, molybdenum,2 nickel, selenium, and zinc. Standards for five of the currently regulated chemicals (arsenic, cadmium, lead, mercury, and selenium) are based on potential adverse human health effects. Most standards are only for eight chemicals; only a ceiling concentration is currently established for molybdenum, as described in the footnote.
In December 1999, EPA issued a proposal to amend the Part 503 rule for land-applied biosolids by adding a risk-based concentration limit for dioxins, a category of organic compounds that includes 29 specific congeners of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and coplanar polychlorinated biphenyls (PCBs) (EPA 1999b). (More details about this proposal are presented in Chapters 2 and 5.)
EPA established operational standards for pathogens in biosolids rather than risk-based standards, although it conducted a preliminary set of risk assessments for viruses (EPA 1992c), bacteria (EPA 1991a), and parasites (EPA 1991b). The operational standards are pathogen-reduction requirements, the goal of which is to reduce the presence of pathogens (including
enteric viruses, bacteria, parasites, and viable helminth ova) in biosolids to levels that are unlikely to pose a threat to public health and the environment under specific use conditions. Because of the variety of different pathogens that might be present in sewage sludge and the impracticality of testing for all of them, EPA requires analyses of “indicator organisms.” An indicator organism is a particular species of microorganism whose presence is used to indicate that a certain set of pathogenic organisms might also be present. The Part 503 rule specifies operational standards for fecal coliforms, Salmonella sp. bacteria, enteric viruses, and viable helminth ova.
Earlier NRC Review
In 1996, the NRC published the report Use of Reclaimed Water and Sludge in Food Crop Production, which reviewed the practice of using wastewater and biosolids for agricultural purposes. That report focused specifically on issues related to food-crop production and evaluated the regulations for chemicals and pathogens in the Part 503 rule; reviewed the impacts on soil, crops, and groundwater; and considered the economic, legal, and institutional issues of the practice. The current report is different from the earlier one in that it encompasses all land-application uses (not only food-crop production), is focused only on human health risks, and provides an in-depth assessment of the methods used to assess those risks.
The 1996 report concluded that “While no disposal or reuse option can guarantee complete safety, the use of [municipal wastewater and biosolids] in the production of crops for human consumption, when practiced in accordance with existing federal guidelines and regulations, presents negligible risk to the consumer, to crop production, and to the environment. Current technology to remove pollutants from wastewater, coupled with existing regulations and guidelines governing the use of reclaimed wastewater and sludge in crop production, are adequate to protect human health and the environment.” However, the report also highlighted limitations and inconsistencies in EPA’s risk evaluation and made recommendations for additional research. Excerpts of the major recommendations of that report are presented in Box 1–2.
One of the major concerns with respect to EPA’s risk evaluation was the reliability of the National Sewage Sludge Survey (EPA 1990), which served as the basis for many of the decisions made in the Part 503 rule, including EPA’s decision to exempt organic pollutants from regulation. Inconsistencies were found in the survey’s sampling and data-reporting methods that undermined the reliability of the data. Therefore, it was recommended that EPA conduct another national survey of pollutants in biosolids. To date, no comprehensive survey has been performed.
BOX 1–2 Recommendations in NRC (1996) Report
Adequacy of Existing Regulations for Pathogens in Reclaimed Water and Biosolids
Adequacy of Existing Regulations for Harmful Chemicals in Reclaimed Water and Biosolids
Marketing Biosolids Products to the Public
Soil, Crop, and Groundwater Effects
tion will be needed to assess potential, long-term impacts of biosolids on groundwater quality and on the sustainability of soils for crop production.
Economic, Legal, and Institutional Issues
The 1996 NRC report also examined the adequacy of EPA’s pathogen requirements and made recommendations to improve them (Box 1–2). EPA3 has indicated that it plans to develop better analytical protocols for detecting pathogens, including Salmonella, as resources permit. It notes that, in general, most biosolids producers continue to demonstrate Class A quality by relying on the fecal coliform tests rather than the Salmonella test. EPA also plans to develop monitoring protocols for specific pathogens.
EPA3 has not decided whether to reevaluate the 30-day waiting period required before grazing is allowed on biosolids-amended pastures. A decision will be based on EPA’s review of a workshop held in June 2001 titled Emerging Pathogen Issues in Biosolids, Animal Manures and Other Similar By-products and a microbial risk-assessment model currently being developed by researchers at the University of California at Berkeley for the Water Environment Research Foundation.
HUMAN HEALTH AND RISK-ASSESSMENT ISSUES
A number of potential human health and risk-assessment issues were brought to the committee’s attention. Some of the major human health issues include the following:
Differences in the extent of health complaints. There are several allegations of deaths caused by exposure to biosolids and anecdotal reports of illnesses ranging from acute to chronic problems, including headaches, respiratory problems, and gastrointestinal illnesses. Most health complaints appear to be concentrated in specific locales. Other locales receive few or no complaints.
Citizen complaints. Odors from biosolids are the principal complaint from citizens living near biosolids land-application sites. Citizens have also complained of attraction of vectors (e.g., insects, birds), declines in property values, and damage to property and public roads by the heavy trucks used to transport biosolids. These types of complaints have sometimes been categorized as nuisance problems or aesthetic issues, but concerns have been raised that odors and vector attraction could have health impacts.
Differences in public confidence in enforcement and compliance with the Part 503 rule. A variety of alleged incidents were brought to the committee’s attention, including improper application of biosolids, inadequate public-access restrictions at Class-B application sites, and violations of the 30-day waiting period before allowing grazing on treated pastures. It was beyond the scope of the committee’s task to investigate or verify these allegations, but an audit of the national biosolids program by EPA’s Office of Inspector General concluded that “EPA does not have an effective program for ensuring compliance with the land application requirements of Part 503. Accordingly, while EPA promotes land application, EPA cannot assure the public that current land application practices are protective of human health and the environment” (EPA 2000).
In addition to health issues, questions have been raised about the risk-assessment approaches used to establish the biosolids standards. Major issues include the following:
Regional and site-specific considerations. Biosolids content, use practices, and application-site characteristic (e.g., geology and climate) vary greatly among and within regions. It is important that these variations are considered in the risk assessment used to establish the biosolids standards.
Difficulties in conducting risk assessments when the available database is poor. Major gaps in the biosolids data include need for updated characterization of biosolids constituents, exposure information, and understanding of relevant health effects.
Challenge of assessing risks from a complex mixture. Biosolids are a mixture of organic and inorganic chemicals and biological agents. Risk-assessment procedures typically quantify risks from single chemicals and assume additivity when multiple chemicals are present. Although much thought has been given to evaluating risks from chemical mixtures, strategies for considering risks from exposure to complex mixtures are still in development.
THE COMMITTEE’S TASK
The Clean Water Act requires EPA to periodically reassess the scientific basis of the Part 503 rule, including the option of adding pollutants to the regulation. Several advances and improvements in conducting risk assessments have occurred since the promulgation of the rule in 1993. Some researchers have questioned the scientific basis and data used in establishing EPA’s biosolids standards, noting data gaps, nonprotective policy choices, and
more stringent standards set by other countries. In addition, there is increasing concern among communities near land-application sites about the health risks from exposure to biosolids. For these reasons, EPA asked the NRC to conduct an independent evaluation of the technical basis of the Part 503 rule land-application standards.
In response to this request, the NRC convened the multidisciplinary Committee on Toxicants and Pathogens in Biosolids Applied to Land. The committee was asked to review information on the land application of biosolids and to evaluate the methods used by EPA to assess human health risks from chemical pollutants and pathogens in biosolids. Specifically, the committee was asked to:
Review the risk-assessment methods and data used to establish concentration limits for chemical pollutants in biosolids to determine whether they are the most appropriate approaches. Consider the NRC’s previous (1996) review and determine whether that report’s recommendations have been appropriately addressed. Consider (a) how the relevant chemical pollutants were identified; (b) whether all relevant exposure pathways were identified; (c) whether exposure analyses, particularly from indirect exposures, are realistic; (d) whether the default assumptions used in the risk assessments are appropriate; and (e) whether the calculations used to set pollutant limits are appropriate.
Review the current standards for pathogen elimination in biosolids and their adequacy for protecting public health. Consider (a) whether all appropriate pathogens were considered in establishing the standards; (b) whether enough information on infectious dose and environmental persistence exists to support current control approaches for pathogens; (c) risks from exposure to pathogens found in biosolids; and (d) new approaches for assessing risks to human health from pathogens in biosolids.
Explore whether approaches for conducting pathogen risk assessment can be integrated with those for chemical risk assessment. If appropriate, recommend approaches for integrating pathogen and chemical risk assessments.
THE COMMITTEE’S APPROACH
To accomplish its task, the committee held five meetings between March 2001 and May 2002. The first two meetings involved data-gathering sessions that were open to the public. The committee heard from EPA, the National Institute for Occupational Safety and Health, industry representatives, environmental and community groups, and academics. Many concerned members
of the public attended the meetings and were given the opportunity to address the committee. Citizens living near land-application sites voiced concerns about odors, health effects, lack of investigation into health complaints, and application practices that do not comply with the regulations. At its second meeting, the committee also visited an agricultural field in Riverside County, California, where Class B biosolids were being applied. The purpose of the visit was to observe techniques used to apply biosolids to an agricultural field. The committee also reviewed a large body of written material on biosolids. The committee relied on peer-reviewed publications as its primary source of information, but unpublished data (submitted by various sources, including industry representatives and the public) were sometimes used to supplement existing information or when no other information was available.
The committee is aware that some readers expect this report to cover all aspects of biosolids use and determine whether EPA should continue to promote its use. That expectation goes well beyond the committee’s charge. Therefore, it is important to clarify what this report addresses and what it does not address.
This report focuses on the land application of Class A and Class B biosolids. It does not consider risks from sewage treatment processes (including composting), storage, or transporting, nor does it cover risks from disposal practices of landfilling, surface disposal, or incineration.
The committee was asked to devote its efforts to evaluating existing biosolids regulations (as of July 1, 2000) in 40 CFR Part 503. Because the regulations cover only chemical (specifically inorganics) and pathogenic pollutants, radioactive contaminants were not included in the committee’s assessment, even though the committee is aware that radioactive compounds may be present in biosolids. The committee’s assessment also excluded an in-depth evaluation of EPA’s risk assessment and proposed regulations for dioxins, because they were not finalized at the time of writing. However, the committee did evaluate the scientific basis of EPA’s original decision not to regulate organic pollutants in biosolids.
Although the Part 503 rule considers risks to both human and environmental health, the committee was asked to focus its evaluation on human health risks and not on plant, animal, or ecological risks. The committee interpreted this task to include an evaluation of relevant occupational health, in addition to public health. It is also important to emphasize that the primary purpose of this report is to provide an evaluation of the risk-assessment methods and approaches used to establish the biosolids land-application standards and is not an investigation into the validity of allegations of biosolids-related illnesses. Risk assessment is the characterization of potential adverse health effects resulting from exposure to environmental hazards. It is a process
separate from risk management, which is the term used to describe the process by which risk-assessment results are integrated with other information (e.g., social, economic, and engineering factors) to make decisions about the necessity, method, and extent of risk reduction.
The remainder of this report is organized into six chapters. Chapter 2 describes the history of the biosolids regulations, treatment processes, use practices, compliance issues, and risk-management practices in the United States. It also provides a brief overview of biosolids regulations and practices in Europe. Chapter 3 reviews the available evidence on human health effects from exposure to biosolids. Chapter 4 presents developments in risk assessment since the Part 503 rule was established and discusses current risk-assessment practices used by EPA. Chapter 5 reviews EPA’s risk-assessment approach to setting limits for chemical pollutants in biosolids. EPA’s pathogen-reduction standards are reviewed in Chapter 6, along with new developments in the area of risk assessment for microbial agents. Chapter 7 explores whether it is possible to use an integrated approach to assess the risks from a complex mixture of chemical and biological agents.
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